We intend to study the neural basis of daily activity rhythms in behavior. We have chosen the marine gastropod, Aplysia, as an experimental animal because we have had extensive experience with this organism, because there is a rich backgroung of information and technique in Aplysia neurobiology, and because Aplysia has robust activity rhythms which are experimentally tractable and theoretically interesting. The problem can be broken down into a search for the alleged receptors, circadian oscillators and physiological coupling mechanisms which are the presumed elements mediating activity cycles. The research will be guided by the following questions: (1) Where are the sensory pathways and receptors, especially photoreceptors, by which environmental rhythms drive circadian rhythms? (2) Are the eyes, by themselves, a sufficient sensory basis for photic driving of rhythms? (3) Do photoreceptors exert their influence on rhythms by the entrainment of circadian oscillators, by the direct excitation of sensory kineses, or both? (4) Are the multiple overt rhythms driven to the same degree and in the same manner by a single set of photoreceptors and circadian oscillators? (5) Where are the nervous organs which function as endogenous circadian oscillators? While the individual photoreceptors, oscillators and coupling mechanisms are interesting in themselves, we believe the most worthwhile objective of our study is to learn about the organization of multiple control elements which, taken in their proper relationship, account for the adaptive daily timing of activity and rest.